LangGraph checkpoint packages receive delta-history and expiry fixes

LangChain released langgraph-checkpoint 4.2.0 with an opt-in omit_expired behaviour and a fix for write collection from plain-value seeds in delta history. langgraph-checkpoint-postgres 3.1.2 also fixes plain-value seed discovery and runs the checkpoint conformance suite.

Why it matters: These are small but production-relevant changes for durable, stateful agents. Teams using time-to-live policies or reconstructed checkpoint histories should regression-test replay, expiry and recovery before upgrading.